Information is more rapidly processed and transmitted using optical as opposed to electrical signals. There exists a need for finding nonlinear optical materials, and processes for preparing such materials, which alter the transmission of optical signals or serve to couple optical devices to electrical devices, i.e., electrooptic devices.
Some materials which have been used in electrooptic devices include semiconductors such as lithium niobate, potassium titanyl phosphate and gallium arsenide and most recently, organic materials which have been doped with nonlinear optical materials. Generally, polymeric organic materials can or may have the specific advantages of fast response time, small dielectric constant, good linear optical properties, large nonlinear optical susceptibilities, high damage threshold, engineering capabilities, and ease of fabrication.
There are various known polymeric organic materials which possess specific nonlinear optical properties and various known processes for making such polymeric organic materials. Many of the current polymeric organic materials prepared by the prior art are prepared by blending a NLO molecule into a polymer host material. "Blending" herein means a combination or mixture of materials without significant reaction between specific components.
Nonlinear optical properties of organic and polymeric materials was the subject of a symposium sponsored by the ACS division of Polymer Chemistry at the 18th meeting of the American Chemical Society, September 1982. Papers presented at the meeting are published in ACS Symposium Series 233, American Chemical Society, Washington, D.C. 183. The above-recited publications are incorporated herein by reference.
EP 218,938 discloses one method of making a polymer with nonlinear optical properties by incorporating molecules which have nonlinear optical (NLO) properties into a host polymer. The NLO molecules are incorporated into the host polymer by blending. The NLO molecules in the polymer can be aligned by an electric field while the temperature of the polymeric material is raised above its glass transition temperature and then cooled to room temperature. EP 218,938 discloses a number of polymer host materials, including epoxies, and many types of molecules which have NLO activity including azo dyes such as Disperse red 1.
PCT Application W08802131A also describes a method of blending a substance having nonlinear optical properties, such as 2-methyl-4-nitroaniline, into a commercially available curable epoxy resin polymer to prepare an electrooptical material.
It is also known to incorporate a NLO active group such as azo dye Disperse Red 1 (4,-[N-ethyl-N-(2 -hydroxyethyl]amino-4-nitro azobenzene), by simply blending the azo dye in a thermoplastic material such as poly(methylmethacrylate), as described in Applied Physics Letters 49, 4 (1986). In this paper, an aromatic amine is disclosed but the amine is not covalently bonded to the polymer chain. In addition, the paper discloses an NLO molecule which has an electron donor and acceptor group at either end of the molecule.
A problem associated with a polymer with NLO properties produced by simply blending NLO molecules into a host polymer is that these polymer materials lack stability of orientation, i.e., there is a great amount of molecular relaxation or reorientation within a short period of time resulting in a loss of NLO properties. For example, as reported by Hampsch et.al., Macromolecules 1988, 21, 528-530, the NLO activity of a polymer with NLO molecules blended therein decreases dramatically over a period of days.
Generally, the incorporation of molecular structures which have NLO activity into the backbone of a polymer chain will decrease the likelihood of the structural reorganization in comparison with polymers in which the NLO active molecule is simply blended. It is therefore desirable to provide a polymer material with NLO groups covalently bonded to the backbone of the polymer material to minimize relaxation effects.
U.S. Pat. No. 4,703,096 discloses a polymer composition in which the NLO activity is derived from aromatic structures attached to a polymeric diacetylenio backbone. However, the synthesis of the material described in U.S. Pat. No. 4,703,096 is complicated.
There is a continuing effort to develop new nonlinear optical polymers with increased nonlinear optical susceptibilities and enhanced stability of nonlinear optical effects. It would be highly desirable to have organic polymeric materials with larger second and third order nonlinear properties than presently used inorganic electrooptic materials.
Anisotropic polymeric materials have been shown to be very useful as nonlinear optical media. For second order nonlinear optical properties a net asymmetric orientation is required within the polymer. The fabrication of polymeric materials with NLO properties typically is accomplished by either blending of monomeric dipoles into a host matrix polymer for example, as described in EP 218,938 or by covalently attaching these dipolar functionalities as pendant sidechains for example as described in U.S. Pat. No. 4,703,096. One common dopant or pendant group for polymeric NLO materials is para-nitroaniline. One major problem with NLO materials prepared by blending or having pendant sidechains is that they are susceptible to relaxation effects. Oriented blends or pendant side-chain groups can relax due to thermally activated motions. Only if the activation energy for a relaxation is sufficiently high will the material retain its NLO properties for extended periods of time.
Two possibilities exist for increasing the relaxation time for a polymer. One way is to crosslink a polymer chain to attempt to "lock-in" a particular orientation as described in copending U.S. Pat. application Ser. No. (Attorney's Docket No C-37,729), filed of even date herewith, by J. J. Kester. The other possibility is to have the anisotropic unit be an integral part of the polymer backbone. The present invention is directed to preparing a NLO material by the second approach, viz by making the anisotropic unit integral with the polymer chain in a "head-to-tail" orientation. By "head-to-tail" herein it is meant a polymer derived from a dipolar monomer in which the dipoles are aligned in the same direction, roughly along the backbone axis of the polymer. The advantage of this class of polymers is the higher barrier to statistical randomization when compared with blended or side chain NLO polymers. The result of this is a more thermally stable NLO material. In addition, by making the entire polymer from the molecule containing the NLO group, a higher concentration of NLO groups can be achieved over that obtained by having a NLO guest molecule in an inactive host polymer medium.
The synthesis of polymers having a head-to-tail arrangement in general is not novel. For example, U.S. Patent No. 3,929,742 and 3,371,073 disclose preparing poly(p-benzene)(sulphonamide). Processes for preparing poly(p-benzenesulphonamide) and poly(p-benzamide) are also disclosed in the following three articles: (1) Contreras et.al., "Synthesis of Poly(p-Benzenesulphonamide) Part I Preparation of Sulphonic Acid Derivatives for Use as Intermediates," The British Polymer Journal, December 1980, pp. 192-198; (2) Contreras et al., "Synthesis of Poly(p-Benzenesulphonamide) Part II Solid State Polymerization of Aniline-4-Sulphondicloride via a Sulphone Intermediate," The British Polymer Journal, December 1980, pp. 299-204: and (3) Contreras et al., "Synthesis of Poly(p-Benzenesulphonamide) Part III Solutions Polymerization," The British Polymer Journal, December 1980, pp. 205-211. However, none of the above references disclose a nonlinear optical material having a head-to-tail arrangement.
It is desired to provide a head-to-tail NLO polymer wherein the NLO group is integral to the polymer chain and as such is less susceptible to relaxation effects. It is further desired to provide a NLO polymer which is made entirely of the molecule containing the NLO group and which has a higher concentration of NLO groups than NLO materials having an NLO guest molecule in an inactive host polymer medium.
One object of the present invention is to provide polymer compositions having anisotropic properties which exhibit nonlinear optical effects.
Another object of the present invention is to provide polymers which have enhanced stability of nonlinear optical effects.